1. Field of the Invention
The present invention relates to an antenna unit in which double resonance is realized by using two antennas and in which it is possible to simultaneously change both frequencies. The present invention also relates to a communication device using the antenna unit.
2. Description of the Related Art
Among conventional antenna units, the antenna unit which has been disclosed in Japanese Unexamined Patent Application Publication No. 11-136025 (Japanese Patent Application No. 10-204902) is taken as an example and described with reference to FIG. 8.
In the drawing, an antenna unit 101 is shown. The unit 101 includes a switch 109 is coupled to an antenna 100. The antenna 100 comprises a grounding electrode 103, a radiation electrode 104, a feeding electrode 106, and a control electrode 108 provided on the surface of a basic body 102 made of a dielectric material. One end of the radiation electrode 104 is open circuited. Furthermore, the feeding electrode 106 is formed so as to be close to the open end of the radiation electrode 104 and is connected to a signal source 110. Furthermore, one end of the switch 109 is connected to the control electrode 108 and the other end is grounded.
In the antenna unit 101 thus constructed, the radiation electrode 104 resonates as a microstrip antenna having a line length of .lambda./4 where .lambda. is the wavelength and functions as an antenna when part of the resonance power is radiated into space.
It is possible to change frequencies using the switch 109. That is, when the switch 109 is on, capacitance generated between the open end of the radiation electrode 104 and the control electrode 108 is connected so as to be in parallel with capacitance between the open end of the radiation electrode 104 and the grounding electrode 103. On the other hand, when the switch is off, capacitance is not generated between the open end of the radiation electrode 104 and the control electrode 108. Therefore, when the switch 109 is on, the frequency becomes relatively low, and when the switch is off, the frequency becomes relatively high.
However, in the conventional antenna unit 101, frequencies of the single antenna unit 100 are to be switched over and it is difficult to realize broader bandwidth.